1. Field of the Invention
The present invention relates to improved apparatus and procedures for electrophoretic migration imaging systems which use electrophotosensitive marking particles and more specifically to means and techniques for improving the quality of images produced by such systems.
2. Background of the Invention
Electrophoretic migration imaging processes capable of producing monochromatic or polychromatic images have been extensively described in the patent literature. Early publication of these processes occurred in a series of patents by E. K. Kaprelian including U.S. Pat. No. 2,940,847 issued June 14, 1960; U.S. Pat. No. 3,100,426 issued Aug. 13, 1963; U.S. Pat. No. 3,140,175 issued July 7, 1964 and U.S. Pat. No. 3,143,508 issued Aug. 4, 1964. More recent publications relating to polychromatic, electrophoretic migration imaging processes include U.S. Pat. No. 3,383,393 to Yeh issued May 21, 1968; U.S. Pat. No. 3,384,565 to Tulagin and Carriera issued May 21, 1968 and U.S. Pat. No. 3,384,566 to Clark issued May 21, 1968.
In a typical embodiment of a single-pass, polychromatic, electrophoretic migration imaging system, images are formed by providing a suspension of electrically photosensitive particles of three different color types (each type of particle being sensitive uniquely to a particular color of light) between a transparent, electrically conductive electrode (commonly termed the "injecting electrode") and an electrode bearing an electrically insulating layer on its outer surface (commonly termed the "blocking electrode"). An electric field is applied across the two electrodes while simultaneously exposing the particles to multicolor light image which is selectively absorbed by the particles according to light color.
As these steps are completed, selective particle migration takes place in image configuration producing complimentary images on both electrodes. While the theory of image formation is not completely understood, it is believed that the particles initially bear a charge in the imaging suspension which causes them to be attracted to the injecting electrode upon application of the electric field between the blocking and injecting electrodes. Upon exposure to activating electromagnetic radiation to which they are sensitive (i.e., of a color absorbed), the exposed particles adjacent the injecting electrode apparently undergo a change in charge polarity by exchanging charge with the injecting electrode. These particles, now bearing the same charge polarity as the injecting electrode, are repelled by it and migrate to the blocking electrode. The particles which migrate to the blocking electrode are less able to exchange charge with that electrode's insulating layer and do not therefore readily recycle to the injecting electrode. As a result, an image is formed by particle subtraction on the injecting electrode, such image being typically a photographically positive image, and a complimentary image, typically a negative or reversal image, is formed on the blocking electrode. Electrophoretic migration imaging systems of the type described above are commonly referred to as PhotoElectroPhoretic processes or denoted by the acronym PEP processes.
In U.S. Pat. No. 3,976,485 another approach is disclosed wherein electrically photosensitive particles, disposed between two spaced electrodes having predetermined properties are subjected to an electric field and imagewise exposed to activating electromagnetic radiation and image formation is achieved by immobilizing at least a portion of the exposed photosensitive particles and causing at least a portion of the unexposed particles to undergo a net change in charge polarity. In this approach, termed photoimmobilized electrophoretic recording (PIER), one of the two spaced electrodes has its surface adjacent the photosensitive particles bearing an amount of a dark charge exchange material which provides a net change in charge polarity of photosensitive particles coming in electrical contact therewith in the presence of a field and in the absence of activating radiation. (This surface is sometimes referred to hereinafter as a dark charge exchange layer). The other spaced electrode advantageously may have on its surface adjacent the photosensitive particles, a "blocking" layer, i.e., a layer which under normal process imaging conditions exhibits minimal charge exchange capability with either the exposed or unexposed electrically photosensitive particles.
In accord with various useful embodiments of either of the above-described approaches, the electrically photosensitive particles can be disposed between the spaced electrodes in a liquid imaging suspension comprising an electrically insulating liquid. Upon being admixed in such a liquid, the particles acquire an electrostatic charge of a positive or negative polarity, although it is not uncommon for the imaging suspension to contain a mixture of both positive and negative polarity particles.
In connection with the various known embodiments of the above-described PEP processes, it has been proposed, at one time or another, to modify the customary surface characteristics of the so-called "injecting" electrode and/or the "blocking" electrode. For example, in British Pat. No. 1,193,276 published May 28, 1970 in examples V-IX thereof it is proposed to coat extremely small amounts of a "Lewis base" or a "Lewis acid", such as 2,4,7-trinitro-9-fluorenone, on one or the other of the electrode surfaces used in PEP processes to increase the photographic speed of the PEP process. Similarly, in British Pat. No. 1,347,162 published Feb. 20, 1974 it is proposed to coat a photoconductive layer on the surface of a PEP injecting electrode in an attempt to modify various imaging characteristics of conventional PEP processes, such as photographic speed, D.sub.max., D.sub.min., image contrast, and spectral sensitivity. In Weigl, U.S. Pat. No. 3,616,390 issued Oct. 26, 1971 and Weigl U.S. Pat. No. 3,723,288 issued Mar. 27, 1973 it is proposed to use as a conductive injecting electrode a preilluminated photoconductive zinc oxide-binder coating applied to a conductive substrate. In Weigl, U.S. Pat. No. 3,595,771 issued July 27, 1971, it is proposed to use as the "blocking electrode" of a PEP process a photoconductive insulating coating such as a "charge transfer complex" of a non-photoconductive aromatic polycarbonate polymer and a "Lewis acid", such as 2,4,7-trinitro-9-fluorenone, in an effort to remove accumulated electrostatic charge which may build up on a conventional highly insulating blocking electrode. In Ota and Ota et al, U.S. Pat. Nos. 3,689,399 and 3,689,400 issued Sept. 5, 1972 it is suggested that one can coat an insulating layer on the "injecting" electrode as well as the "blocking" electrode of a PEP image display device. In addition, in British Pat. No. 1,341,690 published Dec. 28, 1971 it is proposed to coat on the surface of an injecting electrode of a PEP process a photoconductive layer, an electroluminescent layer over the photoconductive layer, and a transparent conducting layer over the electroluminescent layer so that exposure may be effected by using the photoconductive layer to selectively energize the electroluminescent layer which in turn emits radiation to expose the image-forming electrically photosensitive particles used in the process.
However, with regard to all of the above-described processes, difficulty has existed in obtaining sufficiently high density in the image formed for utilization, that is, in causing a sufficient quantity of the photosensitive pigment particles to migrate in the desired image pattern onto the utilized electrode surface (the electrode surface on which the subsequently used image pattern is formed).
The exact cause(s) of this difficulty have not been definitely determined; however, certain prior art teachings theorize regarding them and propose improvement solutions based on those theories.
For example, the disclosure of U.S. Pat. No. Re. 28,260 proceeds on the assumption that some particles, bearing charge causing them to be attracted to the injecting electrode, do not obtain a sufficient charge exchange upon exposure and thus remain on the injecting electrode due to small forces bonding the particles to that electrode surface, e.g., Van der Waal's forces. The solution proposed is to provide means for creating a dynamic stress on the weakly retained particles to free them for migration. It is noted that the dynamic stress can be created by a differential linear velocity between a rolling and plate electrode at their imaging interface, and that the velocity differential should not exceed 10% of the velocity at which rolling electrode translates.
U.S. Pat. No. 3,595,772 theorizes that multicolor pigment particles of such imaging suspensions agglomerate and that the inactivated particles of the bound together mass restrain the activated particles therein from desired image migration. The solution proposed is to prestress the suspension to break up the agglomerates.
U.S. Pat. Nos. 3,616,395; 3,737,310; 3,645,874 and 3,850,627 propose improvement of the process by precharging the image suspension to make the particles unipolar and/or to concentrate the particles near the injecting electrode surface prior to the imaging operation.
U.S. Pat. Nos. 3,676,313 and 3,477,934 disclose techniques for precharging the suspension on one of the electrodes to enhance the migration field or neutralize charge which accumulates on one of the electrodes in a continuous operation apparatus.
U.S. Pat. No. 3,595,771 likewise is aimed towards elimination of residual charge (from a previous imaging operation) on an electrode and suggest constructing the electrode of photoconductive insulative material and flood exposing it between imaging sequences.
Thus it is evident there has been a general realization that image quality of photoelectrophoretic migration imaging process could desirably be improved, particularly as to resultant image density, and that a wide variety of specific techniques for obtaining such improvement have been proposed.